This invention relates generally to integrated circuit processes and fabrication, and more particularly, to a precursor and synthesis method, having an allyl-derived ligand, which improves volatility, and which is capable of depositing copper at high deposition rates, low resistivity, and with good adhesion on selected integrated circuit surfaces.
The demand for progressively smaller, less expensive, and more powerful electronic products, in turn, fuels the need for smaller geometry integrated circuits (ICs) on larger substrates. It also creates a demand for a denser packaging of circuits onto IC substrates. The desire for smaller geometry IC circuits requires that the interconnections between components and dielectric layers be as small as possible. Therefore, research continues into reducing the width of via interconnects and connecting lines. The conductivity of the interconnects is reduced as the area of the interconnecting surfaces is reduced, and the resulting increase in interconnect resistivity has become an obstacle in IC design. Conductors having high resistivity create conduction paths with high impedance and large propagation delays. These problems result in unreliable signal timing, unreliable voltage levels, and lengthy signal delays between components in the IC. Propagation discontinuities also result from intersecting conduction surfaces that are poorly connected, or from the joining of conductors having highly different impedance characteristics.
There is a need for interconnects and vias to have both low resistivity, and the ability to withstand process environments of volatile ingredients. Aluminum and tungsten metals are often used in the production of integrated circuits for making interconnections or vias between electrically active areas. These metals are popular because they are easy to use in a production environment, unlike copper which requires special handling.
Copper (Cu) would appear to be a natural choice to replace aluminum in the effort to reduce the size of lines and vias in an electrical circuit. The conductivity of copper is approximately twice that of aluminum and over three times that of tungsten. As a result, the same current can be carried through a copper line having nearly half the width of an aluminum line.
The electromigration characteristics of copper are also much superior to those of aluminum. Aluminum is approximately ten times more susceptible than copper to degradation and breakage due to electromigration. As a result, a copper line, even one having a much smaller cross-section than an aluminum line, is better able to maintain electrical integrity.
There have been problems associated with the use of copper, however, in IC processing. Copper pollutes many of the materials used in IC processes and, therefore barriers are typically erected to prevent copper from migrating. Elements of copper migrating into these semiconductor regions can dramatically alter the conduction characteristics of associated transistors. Another problem with the use of copper is the relatively high temperature needed to deposit it on, or removing it from, an IC surface. These high temperatures can damage associated IC structures and photoresist masks.
It is also a problem to deposit copper onto a substrate, or in a via hole, using the conventional processes for the deposition of aluminum when the geometries of the selected IC features are small. That is, new deposition processes have been developed for use with copper, instead of aluminum, in the lines and interconnects of an IC interlevel dielectric. It is impractical to sputter metal, either aluminum or copper, to fill small diameter vias, since the gap filling capability is poor. To deposit copper, first, a physical vapor deposition (PVD), and then, a chemical vapor deposition (CVD) technique, have been developed by the industry.
With the PVD technique, an IC surface is exposed to a copper vapor, and copper is caused to condense on the surfaces. The technique is not selective with regard to surfaces. When copper is to be deposited on a metallic surface, adjoining non-conductive surfaces must either be masked or etched clean in a subsequent process step. As mentioned earlier, photoresist masks and some other adjoining IC structures are potentially damaged at the high temperatures at which copper is processed. The CVD technique is an improvement over PVD because it is more selective as to which surfaces copper is deposited on. The CVD technique is selective because it is designed to rely on a chemical reaction between the metallic surface and the copper vapor to cause the deposition of copper on the metallic surface.
In a typical CVD process, copper is combined with a ligand, or organic compound, to help insure that the copper compound becomes volatile, and eventually decomposes, at consistent temperatures. That is, copper becomes an element in a compound that is vaporized into a gas, and later deposited as a solid when the gas decomposes. Selected surfaces of an integrated circuit, such as diffusion barrier material, are exposed to the copper gas, or precursor, in an elevated temperature environment. When the copper gas compound decomposes, copper is left behind on the selected surface. Several copper gas compounds are available for use with the CVD process. It is generally accepted that the configuration of the copper gas compound, at least partially, affects the ability of the copper to be deposited on to the selected surface.
Copper metal thin films have been prepared via chemical vapor deposition by using many different kinds of copper precursors. In 1990, D. B. Beach et al. Chem. Mater. (2) 216 (1990) obtained pure copper films via CVD by using (.eta..sup.5 -C.sub.5 H.sub.5)Cu(PMe.sub.3), and later, in 1992, H. K. Shin et al., Chem. Mater. (4) 788 (1992) declared the same results by using (hfac)Cu(PR.sub.3).sub.n (R=methyl and ethyl and n=1 and 2). However, these copper precursors are solids, which can not be used in the liquid delivery system for copper thin film CVD processing. Furthermore, the copper films often contain contamination of carbon and phosphorus, which can not be used as interconnectors in microprocessors.
Cu.sup.2+ (hfac).sub.2, or copper (II) hexafluoroacetylacetonate, precursors have previously been used to apply CVD copper to IC substrates and surfaces. However, these Cu.sup.2+ precursors are notable for leaving contaminates in the deposited copper, and for the relatively high temperatures that must be used to decompose the precursor into copper.
The studies of copper precursors conducted in the early of 1990's were concentrated on the evaluation of a series of copper(I) fluorinated .beta.-diketonate complexes, which have been proven to be very promising sources for the use in the chemical vapor deposition of copper metal thin films. Copper(I) fluorinated .beta.-diketonate complexes were first synthesized by Gerald Doyle, U.S. Pat. No. 4,385,005 (1983) and U.S. Pat. No. 4,425,281 (1984), in which he presented the synthesis method and their application in the separation of unsaturated organic hydrocarbons. In the U.S. Pat. No. 5,096,737 (1992), Thomas H. Baum, et at., claimed the application of these copper(I) fluorinated .beta.-diketonate complexes as copper precursors for CVD copper thin film preparation. Copper thin films have been prepared via chemical vapor deposition using these precursors.
Among several liquid copper precursors, 1,5-dimethyl 1,5-cyclooctadiene copper(I) hexafluoroacetylacetonate mixed with 1,6-dimethyl 1,5-cyclooctadiene copper(I) hexafluoroacetylacetonate ((DMCOD)Cu(hfac)) and hexyne copper(I) hexafluoroacetylacetonate ((HYN)Cu(hfac)) were evaluated in detail. The copper thin films deposited using (DMCOD)Cu(hfac) have very good adhesion to metal or metal nitride substrates, but a high resistivity (2.5 .mu..OMEGA..multidot.cm) and a low deposition rate. (HYN)Cu(hfac) copper film has poor adhesion to a TiN substrate, and high resistivity (.about.2.1 .mu..OMEGA..multidot.cm). Another compound, butyne copper(I)(hfac), ((BUY)Cu(hfac)), gives a copper film with low resistivity (1.93 .mu..OMEGA. cm), but has poor adhesion and is relatively expensive. Also, the compound is a solid and, therefore, difficult to use in a liquid delivery system. The invention of copper(I)(hfac) stabilized with a series of trialkylvinylsilane (John A. T. Norman et al., U.S. Pat. No. 5,085,731 (1992)) improved the properties of copper thin films.
Copper films deposited using a liquid copper precursor, (hfac)Cu(TMVS), where TMVS=trimethylvinylsilane, have low resistivities and reasonably adhesion to substrates. This precursor is useful because it can be used at relatively low temperatures, approximately 200.degree. C. This liquid copper precursor has been used for the preparation of copper metal thin films via CVD for some time, but there are still some drawbacks: stability, the adhesion of copper films, and cost for the trimethylvinylsilane stabilizer. Also, the precursor is not especially stable, and can have a relatively short shelf life if not refrigerated. Various ingredients have been added to (hfac)Cu(tmvs) to improve its adhesiveness, temperature stability, and the rate at which it can be deposited on an IC surface. U.S. Pat. No. 5,744,192, entitled "Method Of Using Water To Increase The Conductivity Of Copper Deposited With Cu(HFAC)TMVS", invented by Nguyen et al., discloses a precursor and method of improving the electrical conductivity of Cu deposited with (hfac)Cu(tmvs).
It is generally acknowledged in the industry that (hfac)Cu(tmvs) becomes unstable, and begins to decompose, above 35.degree. C. Use of a (hfac)Cu(tmvs) precursor stored at this temperature leads to undesirable process results. The effectivity of (hfac)Cu(tmvs) stored at temperatures lower than 35.degree. C. is also unpredictable. A "fresh" batch of precursor, or precursor stored at temperatures well below room temperature, is used to guarantee predictable processes.
A Cu precursor comprising a ligand of methoxy and methyl groups is disclosed in co-pending application Ser. No. 08/779,640, filed Jan. 7, 1997, entitled "Precursor with (Methoxy)(methyl)silylolefin Ligands to Deposit Cu and Method for Same", invented by Senzaki et al., and assigned to the same assignee as the instant patent. The disclosed precursor permits either one or two methoxy groups to be bonded to the silicon atom of the ligand. That is, the precursor can be "fine tuned" using ligands having more methoxy groups than tmvs, but less than tmovs are provided. The oxygen atoms in the methoxy groups contribute electrons to the Cu atoms, to strengthen the Cu-olefin bond, and so, prevent the premature decomposition of the precursor in storage, or as the precursor is heated for the application of Cu to an IC surface. However, only hydrocarbon groups of one carbon atom, CH.sub.3 (methyl) and OCH.sub.3 (methoxy), are disclosed.
A Cu precursor comprising a ligand of alkyl and alkyl groups is disclosed in U.S. Pat. No. 5,767,301, entitled "Precursor with (Alkyloxy)(Alkyl)silylolefin Ligands to Deposit Copper", invented by Senzaki et al. The disclosed precursor describes alkyl groups bonded to the silicon atom of the ligand with alkoxyl groups. However, the search continues for even more effective copper precursors.
Wei--Wei Zhuang et al., in co-pending patent application Ser No. 09/210,099, entitled "Substituted Phenylethylene Precursor and Synthesis Method", filed Dec. 10, 1998, disclose a volatile copper precursor compound comprising Cu.sup.+1 (hexafluoroacetylacetonate), and a substituted phenylethylene ligand including one phenyl group bonded to a first carbon atom. The remaining bond to the first carbon atom is selected from a first group consisting of C.sub.1 to C.sub.6 alkyl, C.sub.1 to C.sub.6 haloalkyl, C.sub.1 to C.sub.6 phenyl, and C.sub.1 to C.sub.6 alkoxyl. A second carbon atom includes a second and third bond. The second and third bonds are selected, independent of each other, from the group consisting of H, C.sub.1 to C.sub.6 alkyl, C.sub.1 to C.sub.6 phenyl, and C.sub.1 to C.sub.6 alkoxyl.
Wei--Wei Zhuang et al., in co-pending patent application Ser. No. 09/215,921, entitled "Substituted Ethylene Precursor and Synthesis Method", filed Dec. 18, 1998, disclose a volatile metal (M) precursor compound for the chemical vapor deposition (CVD) of metal to selected surfaces. The precursor compound comprises M.sup.+1 (hexafluoroacetylacetonate), and a substituted ethylene ligand with a first carbon atom including a first and second bond. The first and second bonds are selected from groups independent of each other. The first bond is selected from a group consisting of C.sub.1 to C.sub.8 alkyl, C.sub.1 to C.sub.8 haloalkyl, and C.sub.1 to C.sub.8 alkoxyl. The second bond is selected from a group consisting of H, C.sub.1 to C.sub.8 alkyl, C.sub.1 to C.sub.8 haloalkyl, and C.sub.1 to C.sub.8 alkoxyl. A second carbon atom includes a third and fourth bond. The third and fourth bonds are selected, independent of each other, from the group consisting of H, C.sub.1 to C.sub.8 alkyl, C.sub.1 to C.sub.8 haloalkyl, and C.sub.1 to C.sub.8 alkoxyl.
Wei--Wei Zhuang et al., in co-pending application Ser. No. 09/281,722 entitled "Alkene Ligand Precursor and Synthesis Method", filed Mar. 30, 1999, disclose a metal(hfac), alkene ligand precursor. The alkene ligand includes double bonded carbon atoms, with first and second bonds to the first carbon atom, and third and fourth bonds to the second carbon atom. The first, second, third, and fourth bonds are selected from a the group consisting of H, C.sub.1 to C.sub.8 alkyl, C.sub.1 to C.sub.8 haloalkyl, and C.sub.1 to C.sub.8 alkoxyl.
As mentioned by Norman et al. in U.S. Pat. No. 5,085,731 (col. 10, lines 34-49), precursors have been made with allyltrimethylsilane (ATMS) that did not form either a volatile liquid or a solid complex. Norman et al. state that "olefinic molecules which simply contain a silicon group not directly attached to the olefinic double bond do not stabilize Cu.sup.+1 (hfac) as effectively as those in which silicon is directly attached to the double bond." However, Man-Young Park et al., in "Chemical Vapor Deposition of Copper Thin Films with (hexafluoroacetylacetonate)Cu(allyltrimethylsilane)", Electrochemical and Solid-State Letters, 1, 1, 32-33, (1998), claim the fabrication of precursors with the ATMS ligand.
It would be advantageous if a copper precursor was found that effectively deposits copper with low resistivity and good adhesion properties. It would be further advantageous if this precursor was inexpensive to synthesize.
It would be advantageous if a method were found of making a Cu(hfac) precursor stable over a wider range of temperatures, and to provide that the precursor remain in liquid phase during storage.
It would be advantageous if a precursor could be developed where silylolefin groups are not directly attached to olefinic double bonds.
It would be advantageous if a precursor could be developed which encouraged a stable Cu.sup.+1 (hfac) bond without the use of silylolefins directly bonded to olefinic double bonds.
It would be advantageous if a metal precursor could be developed which made use of allyl-derived ligands to stabilize the precursor.
Accordingly, a volatile metal (M) precursor compound for the chemical vapor deposition (CVD) of metal, such as copper, silver, and iridium, to selected surfaces has been provided. The precursor compound comprises M.sup.+1 (hexafluoroacetylacetonate) and an allyl-derived ligand. The allyl-derived ligand includes a first carbon atom double bonded to a second carbon atom. The first carbon atom has a first bond to a methylene and a second bond to an hydrogen (H) molecule. The second carbon has a third bond to an H molecule and a fourth bond to an H molecule. The methylene has a further bond to a molecule selected from the group consisting of alkyl, phenyl, trialkylsilane, trialkoxylsilane, halodialkylsilane, dihaloalkylsilane, trihalosilane, triphenylsilane, alkoxyl, halogen, chloroformate, cynanide, cycloalkyl, cycloalkylamine, alkyl ether, isocyanate, pentafluorobenzene, whereby a stable precursor capable of high metal deposition rates is formed.
To improve the stability of the precursor an additive is sometimes added to create a precursor blend. Then, the precursor blend further comprises:
less than approximately 10% substituted allyltrimethylsilane, as measured by weight ratio of the precursor compound, to facilitate a stable liquid phase precursor. Alternately, when some other allyl-derived ligand, besides ATMS, is used to form the precursor, that same allyl-derived ligand is used as the additive. PA1 a) forming a uniformly mixed solution of 12 grams (g) Cu.sub.2 O proportionally to 40 milliliters (ml) of dichloromethane solvent; PA1 b) introducing 16.15 g of ATMS proportionally to the solution of Step a), and forming a uniformly mixed solution; PA1 c) introducing 20 ml of hexafluoroacetylacetone proportionally to the solution of Step b), and forming a uniformly mixed solution; PA1 d) filtering the solution to remove solid material in the solution, whereby excess Cu.sub.2 O is removed; PA1 e) stripping the solution of dichloromethane solvent; and PA1 f) filtering the solution to remove solid material and crystallized compounds, whereby a liquid phase stable precursor is formed.
A method for synthesizing a copper(hfac) allyl-derived precursor has also been provided. Similar processes could be used to fabricate silver or iridium precursors. The method steps for forming Cu(hfac)(ATMS) are:
Specifically, Step a) includes the Cu.sub.2 O, in proportion, of 0.084 mol, in which Step b) includes ATMS, in proportion, of 0.141 mol, and Step c) includes the hfac, in proportion, of 0.141 mol.